Backing for laps



Oct. 20, 1953 'r. G. LEWIS BACKING FOR LAPS Filed May 22, 1950 INVENTOR.

.THOMAS G.- LEWIS A TTORNEY Patented Oct. 20, 1953 UNITED" STATES OFFICE.

BACKING FOR LAPS Thomas G; Lewis, Wilmington, Deli, assignor to E. II du Pont de Nemours: & Company, Wilmin-gton, DeL, a corporationof-Delaware Application May 22, 1950; Serial No. 163,560

3 Claims. (C11 5I204) Thisinventionrelates to backing members for the'support of laps employed in the fine polishing of' metal articles; and particularly to backing members for relatively soft, fiexible'laps such as woven textiles or thin; unitary plastic sheets.

Lapping, as the'term is used in this specification, comprises working a. particulate abrasive material suspended in a liquid vehicle against theisurface'ofa, workpiece until anexceedingly fine, mirror like'finish is imparted'thereto, The objective sought. is the attainment of a very smooth surface finish, while retaining a high degreefof dimensional control, so that the result ing product will. conform to very precise size standards. The lapping of surfaces from their original state to thefinal finish is a progressive operation, involving the use of a series of abrasives ranging from relatively. coarse at the beginning through successively finer sizes to the end. The results secured depend upon-a number. of factors, such as the properties-of. the. abrasive employed, the pressure with which. the abrasive is forcedagainst' the work, the patternof movementpreservedin the contact of the work with the abrasive particles, andother considerations.

This invention is particularly. applicableto the hide lapping, or polishing of metal articles,.by which is intended'improving the surfaceiroma quality correspondingto that where an average surface defect measures about v to 12". micro, inches, as determined byv the conventional rootmean square. method with a profilometer, to a final uality-of. about /2..to microinch R. M. S. In this stage of lappingJthe amount of surface material removed from the work is. relatively slight and greatest attention is .given .to achievinga highly uniform finish. For this reason-,soft cloth, laps of special texture, such as those utilized inoptical-polishing, have proved most-satisfactoryin-this service, and'I. have recently discovered that unique results are obtainedin the finishing of a great number of materials. when polymeric tetrafiuoroethylene is employed as the. fine lapping material, as set forth in my copendingappli'cation Ser'. No. 163,562, filed May 22, 1950".

One object of this invention is'to' provide a backing for soft, flexible laps such that the supporting material will take a shape conforming to the surface of the article worked, whenmoved relativeto the article, but at a slightly retarded rate, so that high spots on the work will be exposed to the" greatest abrading action whereas l'ow spotsw-ill be subjected to'little or no abrasion. Another object of this invention is'to provide whacking for soft,- fiexible'laps which is'selfaccommodatingto theworlc surface, andthere fbre does not require pref'orming before being used for the treatment of articles of different configurations.

Another object of this invention is to provide a backing for soft, flexible laps which is not afifectedby the liquid lappingvehi'cle employed for the suspension ofthe particulate abrasive.

' These and oth'er' objects of 'this invention will become apparent from the following description and the accompanying'drawings in which Figure l is a perspective View of one design of lappinghea'd'which'is provided' with a backing according to this invention, and

Figure 2 is avertical'section taken along'li'ne 2--2 of Figure 1.

Generally, the objects are attainedby backing the flexible lap, the face of which'is forcedinto contact with the work, with a mass of solid bitumen enclosed in a flexible bag substantially impervious to the liquid lapping vehicle, the bitumen having properties of'cold flow such that it willtend to take ashape conforming to the surface of the article. In the course of the slight continuous shifting of the bitumen as the lap is moved relativeto the work, abrasive particles ad- ,iacenthigh spots in the work will'be continually pressed againstthese areas, which will therefore be abraded preferentially. Similarly, abrasive particles adjacent low spots in the work will clear these areas without enlarging or deepening the depressions, the result being that the work surface is polished to a highly uniform finish.

The cold flow properties which the bitumen should possess for best performance are to.some degree dependent. on the lapping pressures and temperatures maintained. The plasticity is reg ulable within limits by incorporating small amounts of a resin, such as rosin or shellac,.t0 increase the hardness, or by 'adding soft waxes, such as beeswax, to soften the material. For lapping pressures below aboutll lbs/sq. in. and operating temperatures ranging from about F; 130100 E, I have found that good results are obtained with the following compositions:

1 to 2%:lbs; coal=tarpitch,suchas that marketed by: the Barrett Company asits Black Diamond grade,

3 to b ess-pine tar, such as that sold-by the-Good Company,

1v to-2:ozs: beeswax, commercial'grade, andl /2; to 4-cc. rectified turpentine.

The ingredients are preferably bodied by heating-the coaltarpitch; pine tar and beeswax'to a temperature of about 450 F., after which the turpentine is added. The mixture is then heated and stirred at 450 F. until the desired plasticity is obtained. The plasticity may be determined by quenching a drop of the material in a water bath maintained at 70 F. After the drop specimen has cooled for 4 or minutes in the bath, it may be tested by shearing between the teeth, and heating of the mixture continued until a consistency is obtained such that the material breaks cleanly and sharply. This test is repeated at 3 to 5 minute intervals during the course of the heating at 450 F., and a proper consistency is ordinarily obtained after 20 to 30 minutes.

A somewhat more precise evaluation of consistency may be achieved by testing the bitumen in the manner detailed in A. S. T. M. specification D 5-25, entitled Penetrometer Test for Bituminous Materials. Using a loading of 197.5

gms. and a holding time of 5 secs. (test temperapolymeric films, the lap comprises a flexible sheet of suitable material mounted on the side of the head facing the piece to be worked. The metal head ll may be constructed in one piece, or may be fabricated from several components bolted or otherwise secured together. The forward face of I l is arcuately recessed to approximate the profile of the work, and the bitumen backing is interposed in this recess between lap I0 and head II.

The bitumen l 2 is preferably cast from the melt into a unitary block about to /2" thick, and of a length and width sufficient to encompass the area it is desired to lap. It is not, however, absolutely necessary that the bitumen be in one piece, since separate pieces adhere into a single mass after only a brief period of operation. The solid block is encased in a cloth bag l3 which is preferably fitted to it rather snugly at the ends, leaving sufficient slack material along the longitudinal edges so that the bag may be clamped firmly to head H by straps l4 secured to the backwardly inclined surfaces by bolts l5. Since some liquid lapping vehicles, such as sperm oil, for example, have a solvent action on the bitumen and therefore tend to alter its plasticity, closely woven canvas bags coated one or more times with shellac or a similar liquid-proofing material are preferred in this service.

Lapping head I I is given a random movement with respect to the work by suitable powered means, not shown, engaging with self-aligning sleeve 13. Lugs I! are provided for the support of the ends of the head by engagement with hanger members constituting a part of the lapping machine proper and therefore not further described herein.

The results obtained in the fine polishing of a large steel casting wheel ft. in diameter and 5 ft. wide using a bitumen backing as hereinabove described emphasize the advantages inherent in this invention. Casting wheels of the type concerned are employed in the manufacture of photographic film, or the like, and must be pro- Vided with a very high quality peripheral surface to safeguard against defects in the product.

In a typical case the outside of the castin wheel is provided with an electroplated copper surface which is lapped to a very high quality finish, and is then provided with an electroplated nickel overcoat which is in turn lapped to the final finish necessary for film manufacture.

The electroplated copper had a hardness ranging from about Rockwell B 75 to Rockwell B 95. The surface was first rough lapped with an arcuately faced tool steel lap of a hardness of about 60 on the Rockwell C scale having a shape conforming to the outside of the wheel and a width slightly greater than the width of the wheel. The steel lap was reciprocated across the work in a path parallel to the axis of the work with a 3" stroke at a rate of 156 strokes/min. and a pressure of 7 to 10 lbs/sq. in., the wheel being simultaneously turned at a speed of about 101 peripheral ft./min. Lapping was initiated with a suspension of 10 ozs. of aluminum oxide of 320 grit size/gal. of sperm oil and treatment continued until a surface quality of 20-30 microinches R. M. S. was obtained, which required about 72 hrs. At this point, 500 grit size aluminum oxide in a concentration of about 10 ozs./gal. of sperm oil was substituted for the 320 grit material and lapping was continued without changing any of the other factors, until the surface quality improved to about 10 to 12 microinches R. M. 3., this condition being reached after 30 hrs. operation.

The work was next fine lapped with an arcuate- 1y shaped tool identical in all respects to that hereinabove described for the rough lapping except that, in this case, a pitch loaded bag was clamped to the face of the tool and the bag overlaid with a wool felt polishing cloth retained in place with the same clamps used to secure the bag in position. The polishing cloth employed was that marketed commercially for optical polishing by the J. I. Morris C0,, Southbridge, Mass, under the trade name Blu-Streak. Polishing was commenced at the same speeds of work and tool as described for the rough lapping with levigated aluminum oxide of 2000 grit size in a concentration of about 10 ozs./gal. of sperm oil, this concentration being reduced progressively to about 6 ozs./gal. as polishing continued. The lapping pressure at the beginning of the polishing was maintained at about 10 lbs/sq. in., which was gradually reduced to about 4 lbs/sq. in. as the surface quality improved. After 48 hrs. of this treatment, a finer abrasive, specifically Linde polishing compound grade B 5125, marketed by the Linde Air Products Co., in a concentration of about 4 ozs./gal. of sperm oil was substituted for the levigated aluminum oxide, and lapping continued at a pressure starting at about 10 lbs/sq. in. and progressively decreasing to about 4 lbs/sq. in. After 12 hrs. a high quality finish varying from about -1 microinch R. M. S. was attained, which constituted an acceptable base for the nickel plated overcoat which was next applied.

The electroplated nickel overcoat had a hardness of about Rockwell B 85. This surface was rough lapped with a tool steel lap in essentially the same manner as the copper and was then polished with a pitch-backed wool-silk polishing cloth (e. g., grade W-S sold by the Morris Co.) employing a 2000 grit size levigated aluminum oxide abrasive in a concentration of about 10 ozs./gal. of sperm oil, which was reduced to about 6 ozs./gal. as the surface improved. Polishing was initiated at a pressure of about 10 lbs/sq. in.

and continued at progressively lower pressures to a final level of about 4 lbs/sq. in. over a period of approximately 50 hrs, when a surface finish of a quality of 1 to 1% microinches R. M. S. was obtained. At this point Linde polishing compound grade B 5125 in the same concentration as hereinabove described for copper was substituted for the levigated aluminum oxide and final polishing was conducted under identical conditions as for the copper undercoat, except that a WS polishing cloth was employed. At the end of 12 hrs. treatment a final surface quality of about /2 to microinch R. M. S. was attained, when polishing was discontinued.

From the foregoing, it will be apparent that exceedingly high quality finishes can be obtained using bitumen backed polishing laps according to this invention. It is to be understood that bituminous materials of relatively widely varying lasticities may be employed for the processing of specific materials at different temperatures and pressures of operation within the scope and spirit of this invention, and the invention is not to be limited except to the extent specifically indicated in the following patent claims.

I claim:

1. In a surface finishing tool, a rigid support arcuately shaped and recessed, and adapted to conform to a cylindrical work surface, a backing support secured thereto comprising a flexible, fibrous bag, and a mass of plasticized bitumen interposed in said recess between the backing material and the rigid support, and having a cold flow plasticity tending to conform to the work surface at temperatures ranging from about 70 F. to 100 F. when moved relative to said work.

2. In the finishing of casting wheels, a rigid support arcuately shaped and recessed, and adapted to conform to the surface of the casting wheel, a, backing support secured thereto comprising a canvas bag impervious to liquid lapping vehicles, a mass of plasticized bitumen enclosed in said bag having a plasticity tending to conform to the surface of said casting wheel by cold flow, at temperatures ranging from about F. to about F., and at lapping pressures applied thereto during rotation of said wheels.

3. In the finishing of the cylindrical surface of work pieces of substantial diameter, a rigid support arcuately shaped to conform to the surface of the cylindrical work piece, a backing support secured thereto comprising a fabric bag, impervious to liquid lapping vehicles, and a mass of plasticized bitumen containing coal-tar pitch and pine-tar pitch interposed between the backing material and the rigid support, having a cold flow plasticity at lapping pressures below about eleven pounds per square inch and normal operating temperatures, and tending to conform to the work surface when moved relative to said cylindrical surface under the lapping operation conditions, to concentrate abrasive particles adjacent high spots on the surface of the cylindrical work piece.

THOMAS G. LEWIS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,666,701 Hill Apr. 1'7, 1923 2,450,433 Leeman Oct. 5, 1948 FOREIGN PATENTS Number Country Date 21,855 Great Britain 1894 OTHER REFERENCES Asphalts and Alliedsubstances, Abraham, 5th ed., D. Van Nostrand Co., Inc., New York, 1945, vol. 1, pp. 123 and 124. 

